The goal of this research is to identify genes and pathways of mammalian host that play a role during Pseudomonas infections. Specifically, the mechanism of action of a class of proteins produced by Pseudomonas aeruginosa, an opportunistic pathogen of immuno- compromised patients, will be studied. These proteins act by distinct intracellular mechanisms in the host cell, but share a common delivery mechanism. Four proteins, exoenzymes S, Y, T and U (ExoS, T, Y and U) are produced by most clinical isolates of P. aeruginosa and are delivered into mammalian cells by a contact-dependent mechanism, called the Type II secretion pathway. Once in the host, these exoenzymes manipulate host signal transduction pathways eventually leading to either host cell cytotoxicity or apoptosis. The host factors that are involved in the delivery and activation of these toxins are not well characterized. This proposal aims to combine the newly developed tools of mammalian somatic cell genetics and the availability of the mammalian genomes to identify host factors that contribute to the function of ExoS, T, Y and U, starting with their export by the bacterial type III secretion machinery to their final site of activity in the mammalian cell cytoplasm.